1. Field of the Invention
The present invention relates to a cylinder block for an internal combustion engine and a process for casting the same.
2. Description of the Prior Art
A cylinder block for an internal combustion engine is produced by a high pressure casting process such as a die casting process. In such a case, a cylinder liner block defining cylinders in the cylinder block is formed with cylinder liners of into a cylindrical shape and mounted in a cylinder barrel portion of a cylinder block body which forms a main portion of the cylinder block (see Japanese Utility Model Publication No. 28289/89).
The conventional cylinder liner block is formed mainly for the purpose of increasing the wear resistance of the cylinder in which a piston slides, but this cylinder liner block does not contribute to an increase in rigidity of the cylinder block itself and particularly to an increase in rigidity of a bearing wall which supports a crankshaft in a crank case portion of the cylinder block.
The conventional cylinder block body is formed into a complicated shape having a cylinder barrel portion including a plurality cylinders, and a crank case portion formed with a plurality of bearing walls for supporting the crankshaft. Therefore, the cylinder block body has both thin and thick portions and hence, it is difficult to make the chilling or solidifying rate uniform over the entire region during solidification of the cylinder block. For example, a base portion of the bearing wall for supporting the crankshaft is formed thick and hence, has a volume larger than those of other portions, thereby bringing about casting defects such as sink marks due to solidification shrinkage effects.
Thus, in order to prevent such casting defects, there has been conceived an approach for partially accelerating the solidifying rate by additionally using a chiller metal portion or other partially chiller means. However, such an approach results in complicated casting equipment and process, thereby bringing about an increase in cost.
Further, in the prior art casting process, in order to form a water jacket directly surrounding an outer peripheral surface of the cylinder liner block and particularly a water jacket having an undercut portion, a core such as a sand core must be used.